1. Field of Invention
This invention relates to lighter-than-air vehicles in general, and to structural plus aerodynamic features optimizing such vehicles for their intended flight environments specifically, with a method to optimize their operation for long-endurance station keeping.
2. Prior Art
The airship field is quite crowded making even modest innovations significant. The need for innovation is most pronounced at very high altitude, here defined as above the jet stream, where many solutions developed for the troposphere fail. For example: traditional design and construction techniques are too heavy; fabrics quickly fail in the intense radiation environment, primarily ultraviolet; traditional shapes are aerodynamically inefficient at the very low Reynolds Numbers above the jet stream [Re: a dimensionless ratio of the product of air density, velocity and length divided by viscosity. Lower Re conditions are more difficult for any aircraft and, therefore, require precise optimization.]; known propulsion systems are inefficient or even ineffective; vehicles cannot be resupplied; and the jet stream's extreme conditions must be crossed at least twice per mission in common latitudes.
On the other hand, flight above the jet stream offers significant advantages, including: generally lower average wind velocities between about 60,000 and 80,000 feet above sea level; the field of view is several hundred miles in diameter; a significantly extended exposure to the energy in sunlight relative to lower altitudes; little to no exposure to extreme weather; significantly extended line of sight contact with low altitude satellites; and there is no interference with or from conventional aircraft.
Given this crowded field, I'll summarize only the recent advances with non-exclusive emphasis on high altitude flight.
The American Institute of Aeronautics and Astronautics, Inc (AIAA) published an excellent summary by Shafer, Kuke, and Lindstrand in 2002. This is essentially the most current work on very high altitude airships. Their first design, “Lotte”, used the conventional elliptical shape appropriate for low altitude plus conventional fins, ballonets [Dictionary.com: “One of several small auxiliary gasbags placed inside a balloon or a non-rigid airship that can be inflated or deflated during flight to control and maintain shape and buoyancy.”], a propeller and lifting gas cells.
Their second model, “Speedy”, still displayed serious problems. It retained conventional, non-rigid construction requiring fins for flight control, for example. A boom extending down from the nose balanced the ship which obviously cannot remain on an operational design. Speedy does display a more relevant shape, however, which is more appropriate for low Re than other designs. The paper focuses on the flight environment and potential missions before suggesting a solution based on solar energy with fuel cell storage.
Shafer et. al. utilized the shape optimization work published by Lutz and Wagner in “Numerical Shape Optimization of Natural Laminar Flow Bodies”. These researchers defined airship shapes optimized for various low Re conditions. Incorporating a turbulent boundary layer, such as that optionally created by my external frame, has not been disclosed previously. While such turbulence increases form drag, it reduces aerodynamic or “wake” drag in a manner similar to the overall benefit produced by dimples on a golf ball. This benefit is more pronounced on relatively smaller airships.
Prior Art: Airship Construction
General construction philosophy is the most crowded area in the airship art. Nearly all airship designs are variations on the pressurized “blimp” defined by Dictionary.com as “A non-rigid, buoyant airship.” Non-rigid designs are outdated and especially inappropriate at high altitudes. They require flexible coverings, typically fabrics or films, to bear stresses that, with safety factors, reach 1000 psi. Systems must be hung from fabric surfaces or on a keel that is itself affixed to fabric or film.
Non-rigid airships also necessarily reduce the buoyancy of lifting gasses by compression. Buoyancy is directly proportional to the weight of a substance contained in a space. Compressing gas, with or without ballonets, is necessary to maintain a non-rigid ship's form in the same way that a party balloon requires internal pressure to maintain shape. But such compression within the same space increases the gas' density which increases the total weight of the artificially dense gas, thereby reducing buoyancy. This problem is not factored into lifting gas calculations.
The recent art in non-rigid design has generally focused on gas management, specifically ballonets.
Swearingen et al, in U.S. Pat. No. 6,837,458 Jan. 4, 2005, uses conventional gas bags and construction formed into an airfoil-shaped airship optimized for passengers and low altitude. Given that its major axis is perpendicular to the direction of flight, control in disturbed air will be problematic.
Ogawa et al, in U.S. Pat. No. 6,698,686 Mar. 2, 2004, describes a means to move lifting gas between lifting-gas cells for pitch control while bulkheads and air filled chambers maintain the airship's shape. But aerodynamic transients act far too quickly to rely on moving huge volumes of gas for balance, especially given the “plurality of additional compartments”, i.e. baffles which inhibit gas movement.
Perry et al, in U.S. Pat. No. 6,609,680 Aug. 26, 2003, disclosed a flaccid launch method that relies on natural lifting gas expansion to fill out the non-rigid ship as it rises to operating altitude plus moving ballast for rotation to a horizontal flight attitude. The ship then destroys the hull to collapse it for descent, with or without its payload. But this design recently failed several times during flight testing because the flaccid launch method does not allow for an optimized shape, resulting in high aerodynamic drag. The propulsion system could not resist the prevailing wind at the operating altitude. And, the thin film hull was once destroyed by environmental forces. This design also prohibits adhering systems to the hull, such as solar panels. Even if the drag versus thrust problems were solved, this technique prevents fully inflated systems checks before launch allowing hidden problems to manifest after it's too late to correct them.
Perry et al, in U.S. Pat. No. 6,607,163 Aug. 19, 2003, disclosed another conventional design but with liquid ballast. Given that vehicle gross weight is the single most important factor in the high altitude environment, lifting ballast is a serious flaw which subtracts directly from useful payload. This patent also discloses an internal solar cell arrangement covered by a transparent membrane. That solution is impractical given that intense ultraviolet radiation turns suitable materials opaque in the relevant frequencies over the long durations intended for this airship.
Yokomaku et al, in U.S. Pat. No. 6,427,943 Aug. 6, 2002, discloses a common diaphragm design for separating lifting gas from air but with suspension cords added to reduce sloshing. That design seems as reasonable as baffles regarding sloshing but does not address the fundamental flaws inherent in such pressurized airships.
Onda in U.S. Pat. No. 6,305,641 Oct. 23, 2001, disclosed a super pressured high-altitude airship with “no in-flow or out-flow of gas”. That design cannot work given that gas expands 16 fold between sea level and the 20 km design altitude. Even if a miraculous material could be developed to withstand that pressure, the vehicle would never reach high altitude. Gas lifts because it is less dense than the surrounding air. Containing all gas in this manner guarantees that the ship will rise no higher than the point at which the highly compressed gas plus the ship's structure weighs essentially as much per volume as the surrounding air.
Campbell in U.S. Pat. No. 5,645,248 Jul. 8, 1997 disclosed a spherical airship of conventional construction with ballonets and ballast. He shows an innovative propulsion system consisting of a duct through the entire ship with a propulsion fan and rudders mounted internally. His objective is to minimize drag within the duct with no mention of aerodynamic drag from the sphere. Given the geodesic frame required to support the internal features, the ballast and the lack even of the boundary layer control taught by Colting '523 below, it's apparent that Campbell '248 does not mention stratospheric flight because the vehicle would be too heavy and create far too much aerodynamic drag to operate there. It would also prove seriously inefficient at any altitude. A sphere is easy to build but impossible to optimize aerodynamically.
Mellady in U.S. Pat. No. 5,538,203 Jul. 23, 1996, expands the normal pair of ballonets into a plurality, again, without addressing the fundamental flaws of non-rigid airships briefly described above.
There has been some attempt at developing semi-rigid airships with a spine or keel.
Nakada in U.S. Pat. No. 5,348,254 Sep. 20, 1994 disclosed a single keel. In this case, the keel does not give shape to the airship but rather is suspended from conventional gas bags and ballonets. Nakada '254's innovation separates the heavy systems from direct attachment to fabric. The keel supports the empennage, gondola and hydrogen motor. This innovation is minor, however, in that problems attaching motors, etc. to external covering fabric are simply replaced by problems attaching a weighted keel to that same fabric.
Hamilton in U.S. Pat. No. 6,708,922 Mar. 23, 2004, disclosed a sectional spine necessary for the modular airship construction, which is his objective. Such a ship must have uniform or symmetric segments by definition and so cannot be optimized aerodynamically. It is relatively heavy given the frame construction with redundant connection points. The result is a conventional, low altitude airship of somewhat reduced efficiency. As in Nakada '254, the spine does not support the shape in this overpressure, non-rigid ship but is necessary for another purpose, to provide connection points for modular segments.
Sanswire Networks LLC has attempted to build a rigid frame airship during 2005 and 2006 for flight above the jet stream as disclosed on http://www.sanswire.com/stratellites.htm. Those designs have failed to date because their conventional rib and bracing design, conventional propulsion, and pressurized gas containers proved too heavy and aerodynamically inefficient for operation. They have yet to fly a relevant airship.
Prior Art: Aerodynamics
There have been attempts at improving or optimizing airship aerodynamics. Significantly, I could not find high altitude or “stratospheric” airship proposals that build on research into the low Reynolds Number (Re) conditions appropriate for such low ambient pressure applications. Relevant research by Lutz and Wagner was discussed above and forms the basis for optimized airship shapes.
Colting in U.S. Pat. No. 6,966,523 Nov. 22, 2005, advocated another spherical airship, this one intended for the stratosphere. He included a pusher propeller at the rear of the ship to draw the boundary layer around the skin by suction and reduce drag. But a round shape is so inherently high drag that there is no practical propulsion system which could overcome such resistance and still be reasonably lifted to 60,000 feet with more than a few hours' endurance. Compounding the problem, boundary layers separate at the widest point on a ship in the very low Re found above the jet stream. In other words, a spherical ship's boundary layer at very low Re will separate long before it could be influenced by the external suction of this design. The result is a propulsion system rendered ineffective by the separated, turbulent airflow at the back of the sphere.
Rist in U.S. Pat. No. 6,311,925 Nov. 6, 2001, discussed a low altitude, very heavy lift hybrid airship that requires the high thrust of turboprop engines to create the necessary forward velocity for its wings to lift the ship. In Rist '925's innovation, wings are required as the ship needs both static and dynamic lift. The result is a lifting hull that's intentionally far too small to create even neutral buoyancy. Rist '925 also taught an external storage chamber for lifting gas that must be carried in a detachable cargo container. That lifting gas movement modulates pitch similar to Ogawa et al, '686; an idea discredited by the rapidity with which atmospheric forces act. The ship is also burdened by conventional ballonets. It is not a true lighter-than-air vehicle and cannot function at significant altitudes.
Lee et al, in design Pat. D427,137 Jun. 27, 2000, appears to disclose a relevant outline in that the general shape would be efficient at a low Re. The ship's aft end includes a pusher propeller contained in a full circumference shroud. But, as in a design discussed later, such a shroud is ineffective. While somewhat reducing drag from detached airflow within the shroud, total drag is not reduced because the detached airflow drag from the ship's after body is simply replaced by drag on the outside surface of the shroud. The shroud's dead weight is an unnecessary burden in addition. This design is also not detailed enough to reveal a means to control the ship.
Chapman in U.S. Pat. No. 6,082,670 Jul. 4, 2000, proposed drag reduction means for fluid borne vehicles. In this case, he provides a full circumference inlet essentially at the ship's widest point that draws fluid into the ship and through the propulsion system. Chapman '670 specifies an afterbody angle of no more than 15 degrees from horizontal. But the substantially full circumference inlet, certainly when combined with the shallow after body, cannot work at low Reynolds Numbers as the fluid boundary will separate immediately aft of that inlet in a manner somewhat similar to Lee et al, 'D137. The weight and complexity of that inlet design will not be offset by a more beneficial drag reduction in air, though it may be beneficial at the very high Re in water. In addition, Goldschmied demonstrated essentially Chapman '670's concept in a wind tunnel before 1981. Goldschmied noted that the full circumference inlet design would only work for one set of conditions, one Re, and so is not practical on a flight vehicle. Further proof that Chapman '670's innovation could only apply at low altitudes is seen in the preferred embodiment which places the ship's “largest diameter” in the forebody while the inlet intended to prevent boundary layer separation appears to be half of the ship's length further aft. Those two points, largest diameter and boundary layer separation, move closer together as altitude increases.
Onda in U.S. Pat. No. 5,358,200 Oct. 25, 1994 attempted boundary layer control in a similar manner. Rather than draw the boundary layer through an internal propulsion system, Onda '200 advocated a braced shroud enclosing the propeller aft of the main body and forward of an inflated empennage. That means is also ineffective for the same reason as Lee 'D137 and Chapman '670: boundary layer separation on the shroud creates essentially the same drag as on an unmodified ship. In addition, Onda '200 added a very large empennage which is itself a significant drag on a vehicle in low Re. Onda also proposed positioning the ship's maximum diameter as far aft as possible. But that simple design criterion too is inefficient as each combination of airship length and operating altitude—the major factors in a Reynolds Number calculation—results in a somewhat different optimum shape.
Prior Art: Sub-Systems
Sub-systems, those significant elements required for operation, taught in several patents seem relevant before close examination.
Drucker in U.S. Pat. No. 6,766,982 Jul. 27, 2004, disclosed another ship with a duct from front to rear (like Campbell '248) but would install a wind turbine in the duct for energy production. Such an airship relies on other propulsion means. Drucker '982 advocated floating with the wind for movement over the ground while extracting energy from that wind. This system cannot work. Vehicles floating with the wind have, by definition, no “relative wind” flowing around them because the vehicle and the air mass travel at the same speed (see unpowered balloons). The wind turbine would not turn without a velocity difference. When driven by the ship's propulsion system, drag from the duct's walls and the turbine would cost more energy than could be extracted given the laws of thermodynamics.
Lee et al, in U.S. Pat. No. 6,425,552 Jul. 30, 2002, disclosed a thermal management system to smooth out buoyancy fluctuations due to heating or cooling of the lifting gas. This proposal would work by storing hydrogen during the day and converting it to heat in fuel cells at night. Lee '552's very complicated system imposes severe design restrictions: the ship must use hydrogen, must have fuel cells, must include pressurized lifting-gas cells plus ballonets, must store water, and must insulate the ship from the sun and albedo radiation. This patent also proposes to heat lifting gas during the day and cool it at night, a process that would exacerbate the same natural heat during the day and cool at night cycle. Another fault is seen in this proposal's plan to control pitch through “cyclically manipulating” temperature. As with the above proposals to shift lifting gas, this method results in a far too slow response to environmental upsets such as turbulence. Such use of heat to maintain pressure and thereby control buoyancy is not useful in this case due to the heavy equipment required. Such an airship would either be massive or have little useful payload. More fundamentally, heating a gas only produces lift if the gas is free to expand. Heating a gas confined in a pressurized cell accomplishes nothing while increasing stress on the cell's structure.
Onda '641 also discussed a duct from the front to the rear of a ship to remove heat from the solar cell area, thereby improving the solar cell's efficiency. But convection, as distinct from conduction, is extremely inefficient in thin gas such as the stratosphere. In other words, moving transient ambient air below the solar cells will not significantly affect their heating in sunlight. Onda '641 does not block such airflow at night thereby denying those solar cells the heat absorbed by the rest of the ship. The result would be solar cells cooled to −60° C. by morning thereby putting tremendous thermal strain on the mechanics.
Onda '641 also states that the air discharged rearward by the ventilation system would reduce drag through fluid control. But Onda never explains how such drag reduction would occur or how he would compensate for the resulting asymmetric forces. Testing on the experimental fixed-wing aircraft X-21A proved that such fluid control requires pulling the boundary layer through the skin. Blowing air from the aft end of the ship, as was disclosed here, would be ineffective as the boundary layer will separate well forward of that point.
Onda '641 also disclosed another weight shifting pitch attitude control system. This proposal has less merit than appears at first glance given the non-rigid design. Shifting batteries as proposed, for example, is impractical to the point of hazardous to the ship's structure when the primary support is fabric. This point is especially relevant given the critical nature of fabric pressurized by the 16-fold expansion of gas. Such fabric cannot survive the slightest imperfection or impact, such as from moving machinery.
Campbell '248 requires a geodesic frame to support the spherical airship. But a geodesic frame is uniquely associated with spheres or sections of a sphere. That design will not work in other shapes. The frame also does not obviate the need for non-rigid airship systems. Campbell '248 still requires pressurized gas bags, ballonets and ballast. It is the worst of both worlds.
Wurst et al, in U.S. Pat. No. 5,518,205 May 21, 1996, advocates a hybrid high altitude aircraft composed largely of helium filled sections. The aircraft is intended to fly above the weather while avoiding winds that substantially exceed its velocity. Those conditions only exist above the jet stream or higher than approximately 55,000 feet in northern latitudes. At those altitudes, this aircraft would experience extreme difficulties. Its weight shifting attitude control would need to rotate the solar cell-covered wing to nearly vertical early and late in the day to meet the requirement to be somewhat normal to the sun's angle of incidence. That severe bank leaves the question of directional control entirely unanswered. The cables supporting that gondola, the gondola itself, and the numerous complicated shapes generate tremendous drag relative to the reduced thrust available from propellers at those altitudes. The hydrogen storage cells towed behind the aircraft would almost never float directly in line and, therefore, would pull the aft of the aircraft up or down as gas is pumped in or out. Such flaws explain why this hybrid aircraft is not in operation.
Takahashi et al, in U.S. Pat. No. 5,071,090 Dec. 10, 1991, disclosed a propulsion and control means requiring a fluid pathway or duct running the full length of a ship from bow to stern. The pathway includes one or more intersections allowing lateral pathways to exit the ship at various points thereby thrusting the front or rear in desired directions with propelled air. But the preferred embodiment cannot be built. Takahashi et al, '090 lists dimensions of “5 to 10 m” length and the same width. A 10 meter long ship of any width, with approximately half of its internal volume devoted to an empty passageway, cannot lift the claimed 1 to 10 horsepower propulsion system with a power supply, much less a gondola and crew cabin. Even assuming a correct configuration, adding structural weight while eliminating approximately half of the space usually available for lifting gas relegates this design to inefficient, low altitude operations with less utility than decades old alternatives.
It is obvious from this review of recent innovations that there is room for improvement in lighter-than-air vehicles, especially those designed for flight above the jet stream. The prior art clearly shows that lighter-than-air vehicles cannot be optimized with the established technologies typified by non-rigid design, control fins and external motors.